Illustrated Glossary of Organic Chemistry

Substitution reaction: A reaction in which any part of a molecule is replaced (substituted).

Some Common Substitution Reaction Types
The moiety in red is substituted by the moiety in blue

Williamson ether synthesis, an SN2 reaction.

Friedel-Crafts acylation, an electrophilic aromatic substitution reaction.

Bromination of methane, a photochemical free radical halogenation reaction.

Fischer esterification, a nucleophilic carbonyl substitution reaction.